Do Marvel trilogies tell a complete story?

Definitely agree. I would also say that these "trilogies" existed for about as long as public interest remained high. The first movie was successful enough to warrant a sequel, and the sequel was successful enough to warrant a second sequel. If the franchise ended up being a trilogy, it probably had much to do with public reception of the second sequel...

Of course there are always exceptions, with LOTR being an obvious example...